(a) Field of the Invention
The present invention relates to a device for adding and dropping optical signals. More specifically, the present invention relates to a device for adding and dropping optical signals of predetermined wavelengths without interaction in the WDM (wavelength division multiplexing) based optical communication systems.
(b) Description of the Related Art
Bulk dielectric optical thin films are widely known as filter elements in optical communication systems. Directions and locations of the dielectric optical thin films fixed in a wavelength filter unit define wavelengths of light transmitted through or reflected from the dielectric optical thin films.
The bulk dielectric optical thin films are used as filter elements that have various characteristics according to substances used for forming the films and the thickness of deposited films. The filter elements include bandpass filters, short-wavelength pass filters, and long-wavelength pass filters.
The bulk dielectric optical thin films generally transmit or reflect optical signals corresponding to specific wavelengths, and accordingly, they are mainly used as bandpass filters.
In the case of three-port wavelength selective filters using the bulk dielectric optical thin films, a waveguide for adding and dropping is required to guide the optical signals corresponding to a predetermined wavelength region reflected by the bulk dielectric optical thin film.
In general, the three-port wavelength selective filter elements transmit the optical signals corresponding to specific wavelengths, and reflect other signals.
Therefore, the three-port wavelength selective filter elements are generally used as bandpass filters or wavelength extraction filter elements.
FIG. 1 shows a brief diagram of a three-port wavelength selective filter element.
As shown, the three-port wavelength selective filter 60 comprises a first focusing lens 611, a collimating lens 612, a second focusing lens 613, a bulk dielectric optical thin film 621, optical fibers 631, 632, and 633, and a dual fiber termination 641/642.
A pair of optical fibers 631 and 632 forms a first dual-fiber termination and are disposed to couple light into and out of first focusing lens 611. In most conventional arrangements, the optical fibers 631 and 632 are affixed within a single unit, such as a ferrule, where the intra fiber spacing is fixed and cannot be adjusted.
An embodiment comprising a second focusing lens 613 and a third optical fiber 633 is disposed beyond a bulk dielectric optical thin film filter element 621 and is used to capture the filtered optical signal passing therethrough.
As shown in FIG. 1, the terminations of the optical fibers 631 and 632, denoted as 641 and 642, respectively, are positioned at the object plane of focusing lens 611. The object plane is indicated by the dashed vertical line designated “O” in FIG. 1. Focusing lens 611 is chosen so as to provide nominal 1-to-1 imaging of terminations 641 and 642 (noted in the inverted forms as 643 and 644) along the image plane, designated by the dashed vertical line “I” in FIG. 1.
When the three-port wavelength selective filter is used as a WDM demultiplexer, the input optical fiber 631 is used as input means for a plurality of WDM signals having different wavelengths.
For example, let it be assumed that optical signals having center wavelengths at λ1, λ2, λ3, and λ4 are injected into the input optical fiber 631.
The optical signals proceed along the input optical fiber 631, and enter a free space in a device 60 through the termination 641.
The four optical signals are passed through the focusing lens 611, and are focused at the point 644 on the image plane I. The signals pass through the point 644 and proceed to the collimating lens 612, which controls the four wavelength division multiplexed optical signals to be dispersed in parallel and be incident upon the bulk dielectric optical thin film 621.
The bulk dielectric optical thin film 621 transmits optical signals corresponding to predefined wavelengths, and reflects optical signals corresponding to other wavelengths. The signals reflected from the bulk dielectric optical thin film 621 are passed back through the collimating lens 612 to proceed in the opposite direction of the incidence direction, and are focused at the point 643 on the image plane I. They are then passed through the focusing lens 611 to focus at the termination 642 of the optical fiber 632, and are transmitted through the output optical fiber 632.
FIG. 2 shows an operation principle of the three-port wavelength selective filter element used as a multiplexer.
As shown, the three-port wavelength selective filter element 70 used as a multiplexer, that is to combine optical signals having different wavelengths onto a single optical fiber.
In this case, fibers 732 and 733 are the “input” fibers and a bulk dielectric optical thin film 721 functions to combine all four wavelengths onto output fiber 731 and propagates (in the reverse direction—that is from right to left) through device 70 so as to be coupled into “output fiber 731. Similarly, λ2, λ3, and λ4 are coupled into fiber 732.
The input optical signal having a center wavelength at λ1 is provided to the optical fiber 733 positioned in the rightmost part, and it passes through the focusing lens 713 to proceed through the bulk dielectric optical thin film 721 and the collimating lens 712, and then to the output optical fiber 731 after passing through the focusing lens 711.
In the same manner, other optical signals having the center wavelengths at λ2, λ3, and λ4 are injected into the optical fiber 732 to proceed. The proceeding optical signals reflect from the bulk dielectric optical thin film 721, and the reflected optical signals are transmitted through the optical fiber 731. Therefore, since the optical signals having the center wavelengths at λ1, λ2, λ3, and λ4 are multiplexed and transmitted through the optical fiber 731, the three-port wavelength selective filter element 70 functions as a multiplexer.
FIG. 3 shows an exemplary filtering device for adding and dropping optical signals utilizing a plurality of filtering unit.
The device 80 includes two separate wavelength selective filter elements 821 and 822 having a multiplexer and demultiplexer function to provide a function of adding and dropping optical signals.
An input fiber 811 has a plurality of separate signals propagating therealong, and is used as the input to the first filter element 821. As will be discussed below, a first filter element 821 is designed to pass the signal propagating at wavelength λ1 and reflect all others. The reflected signals pass a second time through the left hand side of first filter element 821 (as with the arrangement of FIG. 1) and are coupled into a second fiber 813. The second fiber 813 is then used as the input fiber 814 to second filter element 822, where second filter element 822 is designed to pass the signal propagating at λ1 and reflect all others. The reflected waves then propagate back through the second filter element 822 into a third fiber 815.
The method of dropping predetermined channel signals from optical signals of multi-wavelengths through the output optical fiber 812 of the first wavelength selective filter element 821 is referred to as an optical signal dropping function.
When an optical signal having the center wavelength at λ1 is injected into another input optical fiber 816 of the second wavelength selective filter element 822, the second wavelength selective filter element 822 transmits the optical signal corresponding to λ1, and the optical signal proceeds to the output optical fiber 815.
Accordingly, the transmitted optical signals from the output optical fiber 815 comprise all injected optical signals from λ1 to λ4. The method of adding optical signals through the optical fiber 816 and dropping optical signals from the optical fiber 815 is referred to as an optical signal adding function.
FIG. 4 shows a process for adding and dropping the optical signal having the center wavelength at λ2.
Since the device 80 for adding and dropping optical signals uses two three-port wavelength selective filter elements 821 and 822, it has a large amount of additional losses caused by transmission and reflection by wavelength selective thin film filter elements. Also, an additional process such as optical fiber fusion splicing process is needed for connecting the two thin film filters, and the cost multiplies as the size of the filter element enlarges.